1. Field of the Invention
The invention relates to mobile heaters and, more particularly, relates to so-called “flameless heaters” that generate heat without the use of a burner or other flame that is exposed to the surrounding environment.
2. Discussion of the Related Art
Mobile heaters are used in a wide variety of applications to heat the ground, interior spaces such as tents, or equipment such as an oil well heads or fire hydrants. These systems draw ambient air through a plenum, heat the air, and expel the heated air to the worksite. The most common heaters generate heat using an oil or gas powered burner that generates an open flame. These heaters may be either “direct fired heaters” or “indirect fired heaters.” Direct fired heaters heat the air via direct contact with flames from a burner. The heated air and combustion products thus are intermixed. These burners work on the same general principal as a gas grill. In direct fired heat exchangers, exhaust byproducts exchange heat with ambient air before the exhaust byproducts are exhausted from the system in a separate stream from the heated air. These burners work on the same general principal as a furnace.
However, some applications demand a “flameless” heater, i.e., one that generates heat without the use of an open flame that is exposed to the surrounding environment. These applications include those in which the heater is used in or near an environment containing flammable or potentially explosive materials, such as in gas or oil fields. Flameless heaters typically include an internal combustion engine that powers an electric or hydraulic heater.
Mobile heaters, including flameless heaters, often are powered by diesel engines. Operation of such engines under low load conditions for prolonged periods of time can be detrimental to the extent that such operation can lead to hydrocarbon buildup known as “soot.” Soot buildup occurs when unburnt fuel is exhausted out of the engine's combustion chambers and builds up in the engine's exhaust system and valves. The unburnt fuel can accumulate in the engine's Diesel Oxidation Catalyst (DOC) and Diesel Particulate Filter (DPF) and is evident in the form of a dark particulate or oily dust. Soot significantly shortens the life of particulate filters and other engine components, reduces maximum power, and increases emissions.
Soot buildup in engines typically is avoided by periodically implementing a parked or active regeneration procedure. During a parked regeneration procedure, the system is taken off-line as only a small parasitic load is allowed to be applied to the engine, and the engine is run at a relatively high RPM for a period of time sufficient to increase the temperature of the system's exhaust system sufficiently to burn off accumulated soot from the system's exhaust system while fuel is injected into the engine's cylinder during the exhaust stroke. This procedure is performed under control of the engine's electronic control module (ECM), also called the electronic control unit (ECU). The procedure can take 30 minutes or more to complete, necessarily requiring that the machine be taken out of service for at least that period of time. While most machines permit manual override of the parked regeneration procedure, failing to implement the procedure for a prolonged period of time can result in damage to the engine or, at a minimum, the need to replace a relatively expensive diesel particulate filter and diesel oxidation catalyst. An active regeneration procedure also is characterized by the injecting of fuel during the exhaust stroke to burn the excess soot built up in the DPF when the engines ECM determines it is necessary and the exhaust temperatures meet the set criteria. However, an active regeneration procedure is allowed to be performed while the unit is still online and under load. That is, the active regeneration is performed in the background relative to device operation.
Known flameless heaters also are relatively “dumb.” That is, they offer little if any versatility in their outputs. They also are incapable of regulating the system to obtain a designated air outlet temperature.
Still another drawback of known flameless heaters is that they require engine oil changes and related maintenance relatively frequently, typically on the order of every two weeks or every 100 to 250 hours of operation. This requirement for frequent maintenance is highly detrimental since heaters of this type often are used in very remote locations in which maintenance is difficult, if not impossible. It would be preferable to provide a flameless heater that could operate for an entire season, i.e., on the order of 1,000 to 3,000 hours or more, between service intervals.
In addition, most flameless heaters exhibit a “heat rise”, or a maximum temperature increase of air flowing therethrough, of on the order of 90° F. (32° C.). A relatively low heat rise can be detrimental when the machine is operated in extremely cold climates in which subzero Fahrenheit operation is common. Typical advertised temperature rises are 150-180° F. (66-82° C.). It would be preferable to provide the capability of greater heat rises.
The need therefore has arisen to provide a diesel-powered flameless heater that consistently imposes a sufficient load on the engine to reduce or negate the need for parked or attire regeneration to avoid or mitigate soot buildup.
The need also has arisen to provide a flameless heater that can be reliably operated to output a designated, preferably selectable and/or adjustable, output air temperature.
The need further has arisen to provide a flameless heater exhibiting extended service intervals of at least 1,000 hours, and more preferably, of up to 3,000 hours or more.
The need additionally has arisen to provide a flameless heater with relatively high heat rise.
The need additionally has arisen to provide improved methods of operating a flameless heater.